In motor vehicle multi-speed ratio automatic transmissions, it is often desirable to effect shifting from one speed ratio to another without the use of free-wheeling or one-way devices. This requires a coordinated timing control of both off-going and on-coming fluid operated torque transmitting devices in order to achieve a desired amount of overlap as the transmitted torque is shifted from the off-going device to the on-coming device. Typically, the off-going fluid pressure is progressively released through an orifice while fluid pressure is supplied to the on-coming device through a pressure control device such as an accumulator or a servo.
Where a control of the on-coming device engagement rate or stroke time is desired, the fluid flow at the inlet or outlet of the accumulator or servo may be separately regulated. In one known system involving a servo actuated friction band device, for example, a speed-biased regulator valve is used to vary a restriction for the fluid being displaced by the servo piston. In another similar system, an electro-hydraulic valve is pulse-width-modulated at a variable duty cycle to vary a restriction for the fluid supplied to the inlet of the servo. Unfortunately, these systems are relatively expensive to implement and often exhibit some level of supply pressure sensitivity, degrading the ability of the control to achieve the desired engagement rate or stroke time.